1. Field of the Invention
This invention relates to a device and method for separating heavier and lighter fractions of a fluid sample. More particularly, this invention relates to a device and method for collecting and transporting fluid samples whereby the device and fluid sample are subjected to centrifugation to cause separation of the heavier fraction from the lighter fraction of the fluid sample.
2. Description of Related Art
Diagnostic tests may require separation of a patient""s whole blood sample into components, such as serum or plasma, the lighter phase component, and red blood cells, the heavier phase component. Samples of whole blood are typically collected by venipuncture through a cannula or needle attached to a syringe or an evacuated collection tube. Separation of the blood into serum or plasma and red blood cells is then accomplished by rotation of the syringe or tube in a centrifuge. Such arrangements use a barrier for moving into an area adjacent the two phases of the sample being separated to maintain the components separated for subsequent examination of the individual components.
A variety of devices have been used in collection devices to divide the area between the heavier and lighter phases of a fluid sample.
The most widely used device includes thixotropic gel materials such as polyester gels in a tube. The present polyester gel serum separation tubes require special manufacturing equipment to prepare the gel and to fill the tubes. Moreover, the shelf-life of the product is limited in that overtime globules may be released from the gel mass. These globules have a specific gravity that is less than the separated serum and may float in the serum and may clog the measuring instruments, such as the instrument probes used during the clinical examination of the sample collected in the tube. Such clogging can lead to considerable downtime for the instrument to remove the clog.
No commercially available gel is completely chemically inert to all analytes. If certain drugs are present in the blood sample when it is taken, there can be an adverse chemical reaction with the gel interface.
Therefore, a need exists for a separator device that (i) is easily used to separate a blood sample; (ii) is independent of temperature during storage and shipping; (iii) is stable to radiation sterilization; (iv) employs the benefits of a thixotropic gel barrier yet avoids the many disadvantages of placing a gel in contact with the separated blood components; (v) minimizes cross contamination of the heavier and lighter phases of the sample during centrifugation; (vi) minimizes adhesion of the lower and higher density materials against the separator device; (vii) can be used with standard sampling equipment; (viii) is able to move into position to form a barrier in less time than conventional methods and devices; and (ix) is able to provide a clearer specimen with less cell contamination than conventional methods and devices.
The present invention is a method and assembly for separating a fluid sample into a higher specific gravity phase and a lower specific gravity phase. Desirably, the assembly comprises a plurality of constituents. Preferably, the assembly of the present invention comprises a container, a liner and a composite element.
Most preferably, the container is a tube that comprises an open end, a closed end and a sidewall extending between the open end and the closed end. The sidewall comprises an outer surface and an inner surface. The tube further comprises a closure disposed to fit in the open end of the tube with a resealable septum. Alternatively, both ends of the tube may be open, and both ends of the tube may be sealed by elastomeric closures. At least one of the closures of the tube may include a resealable septum.
Most preferably, the liner comprises an open end, a closed end and a sidewall extending between the open end and the closed end. The sidewall comprises an outer surface and an inner surface. The liner is most preferably a thin wall elastomeric material. The liner is positioned in the tube such that the open end of the liner is attached to the inner surface of the tube at the open end and the closed end of the liner is near the closed end of the tube. The liner, in an unbiased condition, is cross-sectionally dimensioned along most of its length to lie in spaced relationship to the tube.
Preferably, the flexible liner comprises a qualitative stiffness that may be characterized by a non-dimensional stiffness coefficient, S* and expressed as follows:       S    *    =            E      ⁢              xe2x80x83            ⁢              (                  OD          -          D                )                    a      ⁢              xe2x80x83            ⁢              ρ        w            ⁢              D        2            
Where E is the modulus of elasticity, OD is the thickness defined by the outside diameter, D is the seal diameter, a is the applied acceleration and xcfx81w is the density of water. The stiffness coefficient is about 0.003 to about 190.
Preferably, the liner has a thickness of about 1.0 mm to about 2.5 mm, a modulus of elasticity of about 13.8 MPa to about 69 MPa.
Preferably, the liner deforms due to hydrostatic pressure under applied acceleration and returns to its initial state upon removal of the acceleration, thereby forming a seal by constricting on a relatively rigid floating member which is positioned in a target density region between the higher density portion and the lower density portion of a fluid sample.
Desirably, the liner may be comprised of any natural or synthetic elastomer or mixture thereof, that are inert to the fluid sample of interest.
Preferably, the composite element is a seal body. The seal body may be a single constituent or a plurality of constituents and comprises a specific density at a target density range as defined by separable fluid components densities.
Desirably, the seal body is a substantially rigid moldable thermoplastic material such as polyvinyl chloride, polystyrene, polyethylene, polypropylene, polyester, marble and mixtures thereof that are inert to the fluid sample of interest.
Preferably, the seal body has an overall density between the densities of two phases of a blood sample.
Preferably, the seal body comprises an overall specific gravity at a target specific gravity of "sgr"t. The target specific gravity is that required to separate a fluid sample into two phases.
Most preferably, the seal body comprises at least one region of a specific gravity However, it is within the purview of the present invention that the seal plug may comprise at least two regions of differing specific gravities whereby at least one of the regions is higher than the target specific gravity and at least one of the regions is lower than the target specific gravity.
Preferably, the seal body may migrate freely when under an applied acceleration to settle at a location in the fluid sample in the target density region and thereby become a barrier at a desired level between the components of the fluid sample after the acceleration is removed.
Preferably, the seal body has an aggregate specific gravity of about 1.028 to about 1.09. Most preferably, the seal body has an aggregate specific gravity so that it will rest after centrifugal force, between the heavier and lighter phases of a blood sample.
Preferably, the seal body is initially secured to the bottom area of the liner by an interference fit until the assembly is subjected to centrifugation. When the assembly is subjected to centrifugation the seal body is released from the bottom of the liner. However, it is within the purview of the invention that the seal body may start at any location in the liner.
Preferably, the assembly of the present invention will function under load created by an applied acceleration of about 300 g to about 3000 g.
In use, a fluid sample enters the assembly by a needle. The needle penetrates the closure through the elastomeric septum and the sample enters the assembly through the needle and into the body of the liner. The needle is withdrawn from the assembly and the septum of the closure reseals.
The assembly is then subjected to centrifugation. Under centrifugation, forces exerted by the centrifuge cause the liner to expand outwardly against the tube, eliminating the interference fit with the seal body and the seal body migrates axially up the liner towards the open end. Therefore, a path is developed between the inner surface of the liner and the seal body that permits the flow of the high-density component past the seal body as it migrates up the liner. The centrifuge may be stopped after the seal body reaches the position between the lower density liquid component and higher density cellular/solid components, equal to its overall density. Upon terminating centrifugation, the liner resiliently returns to its undeformed shape, whereby the seal body seals against the inner wall of the liner, thereby creating a barrier between the higher and lower density components of the fluid. As a result, the phases of the fluid sample are isolated from one another by the seal body and may be separated for subsequent analysis.
The seal body""s position at the bottom of the liner provides easy direct loading of the fluid sample into the liner. Thus, the fluid sample is easily delivered into the liner without any interference or disturbance.
When the fluid sample is blood, the higher specific gravity portion that contains the cellular components is between the seal body and the bottom of the liner after centrifugation. The lower specific gravity portion that contains the cell free serum fraction is between the seal body and the top of the liner. At the final position of the seal body, the seal body is able to substantially eliminate the presence of red blood cells on the seal body in the lower specific gravity portion and the lower specific gravity portion is substantially free of cellular contamination.
The assembly of the present invention is advantageous over existing separation products that use gel. In particular, the assembly of the present invention will not interfere with analytes as compared to gels that may interfere with analytes. Another attribute of the present invention is that the assembly of the present invention will not interfere with therapeutic drug monitoring analytes.
Most notably, is that the time to separate a fluid sample into separate densities is achieved in substantially less time with the assembly of the present invention as compared to assemblies that use gel.
Another notable advantage of the present invention is that fluid specimens are not subjected to low density gel residuals that are at times available in products that use gel.
A further attribute of the present invention is that there is no interference with instrument probes.
Another attribute of the present invention is that samples for blood banking tests are more acceptable than when a gel separator is used.
Another attribute of the present invention is that only the substantially cell-free serum or plasma fraction of a blood sample is exposed to the top surface of the seal body, thus providing practitioners with a clean sample.
Additionally, the assembly of the present invention does not require any additional steps or treatment by a medical practitioner, whereby a blood or fluid sample is drawn in the standard fashion, using standard sampling equipment.